codeowners: dev-multicell owns migration guide

[ryancharris]

What are you changing in this pull request and why?

this pr makes the @dev-multicell team codeowners of the public-facing migration documentation to ensure it reflects the current state of the migration tool (and its limitations).

Checklist

• Review the Content style guide so my content adheres to these guidelines.
• For docs versioning, review how to version a whole page and version a block of content.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

Adding or removing pages (delete if not applicable):

• Add/remove page in website/sidebars.js
• Provide a unique filename for new pages
• Add an entry for deleted pages in website/vercel.json
• Run link testing locally with npm run build to update the links that point to deleted pages

[vercel]

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Name	Status	Preview	Updated (UTC)
docs-getdbt-com	✅ Ready (Inspect)	Visit Preview	Jul 22, 2024 8:04pm

[runleonarun]

Looks good! I willgive the team write access as well!